Moving ListBase, FixedLengthListMixin and UmodifiableListMixin to collection.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index d06776ab2a0b7468f5bc0ce9839786390be56f6c..f659b770e31e4ab709f0ddc1b0bba9e73e31f1a4 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -7684,7 +7684,7 @@ class DomTokenList native "*DOMTokenList" {
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
-class _ChildrenElementList implements List {
+class _ChildrenElementList implements List<Element> {
   // Raw Element.
   final Element _element;
   final HtmlCollection _childElements;
@@ -7742,19 +7742,19 @@ class _ChildrenElementList implements List {
   }
 
   String join([String separator]) {
-    return IterableMixinWorkaround.joinList(this, separator);
+    return _childElements.join(separator);
   }
 
   Iterable map(f(Element element)) {
-    return IterableMixinWorkaround.mapList(this, f);
+    return _childElements.map(f);
   }
 
   Iterable<Element> where(bool f(Element element)) {
-    return IterableMixinWorkaround.where(this, f);
+    return _childElements.where(f);
   }
 
   Iterable expand(Iterable f(Element element)) {
-    return IterableMixinWorkaround.expand(this, f);
+    return _childElements.expand(f);
   }
 
   bool get isEmpty {